NUCLEAR-LOCALIZATION OF P85(S6K) - FUNCTIONAL REQUIREMENT FOR ENTRY INTO S-PHASE

Immunolocalization of a newly described isoform of p70(s6k), termed p8 5(s6k), demonstrated a predominantly nuclear location in rat embryofib roblasts (REF-52), a compartment in which growth factor-mediated phosp horylation of S6 has recently been reported. Microinjection of express ion vectors encoding either p85(s6k) or a fusion protein containing on ly the putative nuclear localization motifs led to the exclusive accum ulation of both products in the nucleus. Consistent with such a locali zation, microinjection of affinity-purified anti-p85(s6k) IgG into the nucleus, but not the cytoplasm, blocked serum-induced initiation of D NA synthesis. Co-injection into the nucleus of the anti-p85(s6k) IgG w ith activated p70(s6k), which lacks the antigenic epitope, rescued the S phase block, arguing that theantibody exerts its effects through in hibiting p85(s6k) function. The results indicate a novel role for S6 p hosphorylation in the nucleus distinctfrom that in the cytoplasm, a ro le essential for mitogenesis.